Thermal management of photonics assemblies

ABSTRACT

A photonics assembly ( 8 - 16 ) is housed within an hermetically sealed container ( 24 ) mounted within an outer container ( 20 ), and is cooled by a fluidic arrangement comprising a liquid flow path ( 28 ) defined between the outer container and the sealed container, the outer container having a liquid inlet and a liquid outlet ( 30, 32 ), whereby cooling liquid can flow around the hermetically sealed container to remove heat. The photonics assembly including photonics devices, has a composite thermally conductive substrate ( 17, 19 ) contacting a thermally conductive wall ( 22 ) of the sealed container, whereby the cooling liquid cools said photonics devices. Liquid flow passageways ( 48 - 66 ) are provided in wall ( 22 ) and substrate ( 17, 19 ) for improving fluidic cooling.

TECHNICAL FIELD

The present invention relates to thermal management of optical and/orelectrical component assemblies, particularly though not exclusivelyphotonics component assemblies.

BACKGROUND ART

The current common method of controlling the temperature of photonicsassemblies, for example one or more photonics devices mounted in acircuit pack, is the use of thermoelectric modules (TE). These are veryinefficient modes of thermal control. Photonic sub-assemblies typicallyoperate at a fixed temperature but are placed within modules and oncircuit packs where they can have a requirement as the lowesttemperature component, which results in heat load being drawn into thepackage. Currently one solution uses a heat spreading material inconjunction with a thermoelectric module. The module is powered tomaintain a set point temperature and localized heating methods areemployed to control specific component or location temperatures. Foroptic components this involves the addition of localized heaters tobring the local component up from the ambient temperature set by thethermoelectric module. Due to the thermal conductivity of the materialsused, thermal crosstalk becomes an issue, resulting in more powerrequired to drop the bulk thermoelectric module temperature. Thermalcrosstalk is where the temperature of one active component affectsanother one. Thermal crosstalk results in larger power consumption bythe thermoelectric modules and laser heaters.

Electronics chips do not usually use TE cooling, since TE cooling isinefficient, and further with electronics chips the main issue is toextract a large amount of heat, rather than controlling chip temperatureprecisely. Common arrangements used for cooling electronics chips arethe use of fluidic cooling and/or fan air cooling.

SUMMARY OF THE INVENTION

Therefore a method suitable for efficiently isolating photonicsassemblies while maintaining a fixed set-point temperature anddissipating the heat load generated is required.

Various embodiments provide thermal management apparatus for a photonicsassembly, comprising an hermetically sealed container mounted within anouter container, a liquid flow path being defined between said outercontainer and said sealed container, and the outer container having aliquid inlet and a liquid outlet, whereby cooling liquid can flow aroundthe hermetically sealed container to remove heat therefrom. Thehermetically sealed container contains a photonics assembly comprisingone or more photonics devices, which are in thermal communication with athermally conductive wall portion of said sealed container, whereby saidcooling liquid cools said photonics devices.

Fluidic cooling has not previously been used for photonics assemblies,because of problems with temperature control and isolation of photonicscomponents from the fluid medium, photonics devices normally beingoperated in an air environment. In accordance with common practice, anhermetically sealed container is provided for protection of opticalcomponents formed of III-V materials and other components. Usually, thesealed container is filled with an inert gas such as Argon or Nitrogen,which also provides a clear optical path for free-standing opticalcomponents.

It will be understood that for the purposes of this specification,fluidic cooling is generally to be understood as cooling by liquid, andreferences to “fluid” herein are to be understood as primarilyreferences to “liquid”. However, two phase cooling is envisaged, whereina liquid is employed having a boiling point such that the liquid boilsinto vapour when performing a cooling action on the hermetically sealedcontainer.

Although embodiments are concerned with photonics equipment, otherembodiments may be implemented for cooling other types of electrical oroptical equipment, for example free-standing optics or equipmentcontaining III-V material, which are required to be housed in anhermetically sealed container.

Said one or more photonics devices may be mounted on a thermallyconductive substrate such as Silicon (Si), which may form a base regionof the sealed container. Alternatively, a thermally conductive baseplate assembly may be provided which is thermally coupled to thephotonics devices, and which either forms a base part of the sealedcontainer, or is in good thermal contact therewith. At least this baseportion of the sealed container is of thermally conductive material suchas metal so as to permit heat to be conducted away by fluid, to which itis exposed, so as to permit the photonics devices to be maintained at adesired temperature. The casing of the sealed container may be formedwholly of metal, that is the base, side walls and top. Alternatively, inone embodiment, the top may be of thermally insulative material andfunctions additionally to form the top of the outer container. The outercontainer is formed as a separate item from the sealed container, aspreferred of a thermally insulating material such as plastics. Thus thesealed container is surrounded by thermally insulating material. This isan added advantage where the containers are mounted on a circuit pack ina “hot” environment where fan air cooling is employed to cool powerconsuming electronics components. The insulating outer casing isolatesthe sealed container from the hot external environment, and the fluidflow conducts away a required amount of heat from the optical devices.Alternatively, in other embodiments the outer container may be formed ofa thermally conductive material, in circumstances where the externalenvironment may provide a cooling effect. The outer container may be acompletely separate item for the sealed container; as preferred it isformed as a container which makes a close fit with the sealed container,or is a plastics extrusion, moulded onto the surface of the sealedcontainer. A top plate is of a thermally insulative material.

The base floor of the outer container in its inner surface may be formedwith fluidic structures or formations to direct fluid flow around thebase region, and avoid the formation of stagnant areas. Alternativelysuch structures may be preformed on a board which is then fitted intothe base of the outer container. In addition, structures may be providedwhich encourage speeding up of fluid flow, by means of the BernoulliEffect, close to the base of the sealed container in regions where hotoptical components are located, so as to provide increased cooling inthose regions. Alternatively, the base of the sealed container, whichwould normally be formed of a sheet having a constant width orthickness, may be more narrow, in relation to the width or thickness ofthe rest of the base, in those regions where hot devices are located, toprovide increased conductance for heat.

In addition, the base of the sealed container may be formed withinternal fluid flow channels, to permit fluid to flow into fluidicpathways and reservoirs formed within a thermally conductive substrateof the photonics components, so that fluid can flow close to specificphotonics components, for example lasers in a laser bar. In addition,for other components such as free standing optics, a small reservoir offluid may be formed close to the optical components, so as to provide agreater amount of thermal control.

Accordingly, there is provided thermal management apparatus for aphotonics assembly, comprising an hermetically sealed container mountedwithin an outer container, and the hermetically sealed containercontaining a photonics assembly comprising one or more photonicsdevices, which are mounted on a thermally conductive substrate assembly,which is in contact with a wall portion of said sealed container, thethermally conductive substrate assembly having liquid passageways formedtherein to permit liquid to flow close to said photonics devices, saidwall portion having inlet and outlet liquid paths communicating withsaid liquid passageways, and the outer container having a liquid inletand a liquid outlet, whereby cooling liquid can flow to said photonicsdevices to remove heat therefrom.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described, by way ofexample only, with reference to the accompanying drawings, wherein:

FIG. 1 is a side view of an existing device containing electronics andphotonics components, with thermoelectric cooling mechanisms;

FIG. 2 is a plan view of the structure of FIG. 1;

FIG. 3 is a side view of a device forming a first embodiment of thepresent invention, including fluidic cooling for cooling a photonic andelectronic component assembly;

FIG. 4 is a plan view of the structure of FIG. 3;

FIG. 5 is an exploded view of the structure of FIG. 3;

FIG. 6 is a side view of a device forming a second embodiment of thepresent invention, including fluidic cooling for cooling a photonic andelectronic component assembly;

FIG. 7 is an exploded view of the structure of FIG. 6 viewed from above;

FIG. 8 is an exploded view of the structure of FIG. 6 viewed from below;

FIG. 9 is a plan view showing the base of an hermetic package of FIG. 6,showing fluid flow connections;

FIG. 10 is a plan view of the structure of FIG. 6 from the undersidewith base components removed to show fluid flow paths;

FIG. 11 is a perspective view of the fluidic path and reservoircomponents of the structure of FIG. 6; and

FIG. 12 is a schematic view of the device of the first or secondembodiments located on a circuit board.

DESCRIPTION OF THE EMBODIMENTS

In embodiments, liquid flow cools the internal components of anhermetically sealed container, while maintaining the hermetic sealrequired for the III-V materials and other internal components. An outercontainer depending on the application can either be a metallicstructure with additional machined features for flow control, or a heatinsulating material such as plastic for lower cost packagingintegration. Whilst the need for a thermoelectric module is avoided,such a module might additionally be employed to improve operatingefficiency. However with fluid flow, an internal thermoelectric moduleneed not be used in the hermetically sealed container, resulting in hugepower savings. A fluid layer is provided around the hermetically sealedcontainer for conducting away heat while not interfering with electricalinput/output pins and optical path connections, which reduces theambient influence on the internal temperature of the hermetically sealedcontainer and also enables liquid cooling of a silicon substrate orother thermally conductive substrate to which the silicon photonics isthermally connected.

In one embodiment of the invention the outer container surrounds thehermetically sealed container and provides a means of bulk heatdissipation, through liquid cooling of the exterior package. The outercontainer has ports for fluid flow but also slots for input/output pinconnectors. The internal hermetically sealed package has a thermal pathto one or more of the other faces allowing for the liquid heatdissipation technique to be employed. Isolation of the internalcomponents from the ambient circuit pack temperature field is donethough the use of an insulating layer and a fluidic heat extractionmethod. The outer structure is a plastic perform with low thermalconductivity but good moisture and solubility repulsion characteristics.The plastic outer structure may also be molded onto the inner packageensuring sufficient sealing at the I/O connections.

One embodiment has an internal surface within the outer container havingformations to control the fluid flow within the outer fluid structure.Protuberance formations are designed to improve the lateral distributionof the flow with minimal pressure drop. These formations ensure asufficient cross section remains for fluid flow over the range ofpossible test pressures. On the outlet fluid side these formations areto control the radial flow ensuring that regions of flow do not becomestagnant or bypassed, reducing the overall effective heat dissipation ofthe system. Such internal surface may be a preformed board which isencapsulated in the plastic outer container, ensuring a good seal,thermal isolation and the correct cross-sectional area for the requiredflow regime.

In some embodiments, localized regions of increased heat transfer areprovided at the base of the package structure. This increased heattransfer is generated by reducing the cross-sectional area(constricting) of the flow hence increasing the velocity. In anotherembodiment localized cooling regions are generated by thinning of theinner package material.

One embodiment of the invention has localized fluid connection points inthe wall of the hermetic package for attaching a network of fluid flowchannels and reservoirs, which are formed within an internal thermalplane, which may be a component substrate, within the hermeticallysealed container. Port openings in the internal package allow for fluidto pass from the bulk system cooling to the internal thermal plane. Theinternal thermal plane includes in its simplest form a number ofchannels within a thermally conductive substrate assembly, along whichthe fluid can pass and return back to the bulk system cooling stream.The sealing of this interface between the thermal plane and wall of thesealed container may be implemented by a number of different methods,such as soldering, non-drip connectors, welding, and adhesive bonding. Apreferred method is a seal by soldering.

Another embodiment uses a two phase fluid for thermal control of theinternal package structure. This approach through preferential densityand surface coatings gives a range of heat transfer locations.

Referring to FIGS. 1 and 2 which show an existing structure containingelectronics and photonics components, the structure comprises a casing2, which forms an hermetically sealed package having at one end anoptical fiber outlet connector 4, and at the opposite end an electricalinput/output terminal pin connector 6. These I/O connectors areas aresealed by a number of different methods, e.g. adhesive bonding,pressurized o-rings, welding, seam sealing.

The hermetic package is necessary on account of the need to provide acontrolled atmosphere for the optics components and to protect III-Vmaterials. A first unit mounted within the housing comprises free spaceoptics and filters 8 mounted on a thermoelectric module structure 8′. Asecond unit comprises optics 10 formed as silicon waveguides (AWG) whichare formed on a circuit card 12, so as to conduct light from a laser bar14, which is mounted on a silicon substrate 16. These items are mountedon a second thermoelectric module 10′. A third unit compriseselectronics components 18, mounted on a thermal spreader, which couplesthe electronics to the hermetic package walls. In operation, electricalsignals applied to terminal 6 are processed in electronics 18 to providecontrol signals to laser bar 14, the lasers therein providing lightsignals which are conducted through the silicon waveguide 10 and freespace optics 8 to output optical fiber 4.

Typically the thermoelectric module 10′ comprises between 100 and 200 ofthermoelectric bumps in an area of 15 mm×15 mm on which silicon heatspreading bar 16, 10×4 mm, is mounted, but other dimensions may beemployed. The laser bar comprises a row of 10 lasers, the lasers beingspaced apart by 250 microns. A respective resistive heater is mountedwith each laser to individually adjust the temperature of the laser. Thepackage shown in FIGS. 1 and 2 is very inefficient at maintainingoptimum operating temperatures. Typically, the operating environment ofan optoelectronics assembly may be around 55° C., whereas the lasersneed to be cooled to an operating temperature of 25° C.

Referring now to the first embodiment of the invention shown in FIGS. 3,4 and 5, similar parts to those of FIGS. 1 and 2 are denoted by the samereference numeral. An outer open container 20 of thermally isolatingplastic material receives a registering base portion 22 of a hermeticpackage 24 (shown in exploded form in FIG. 5), of a thermally conductivemetallic material. A top surface 26 of thermally insulating materialcompletes both the hermetic package, and the outer container. The layoutof the electronic and photonic components is similar to that of FIGS. 1and 2, and it may be seen from FIG. 5 that units 8-18 together comprisea single composite assembly 17, with the conductive silicon substrate ofeach unit 8-18 forming a composite thermally conductive substrateassembly 19. The substrate assembly may be separate substrate units orjoined together units, or be formed as an integral unit.

Fluidic cooling of the components is provided as follows. Betweencontainer structure 20 and base 22 is provided a fluidic passage 28,which permits fluid to flow between a fluid inlet 30 and a fluid outlet32 at opposite ends of the tray. The preferred inlet/outlet is anencapsulated ferrule connection encapsulated in the outer plasticpackaging during formation. The inlet and outlet fluid connectors areremovable and non-dripping. The base 22 has recessed portions 34, 36, 38disposed respectively beneath optics 8, waveguides 12 and laser bar 14,and electronics 18. This permits fluid to flow close, at the uppersurfaces of the recesses 34-38, to the underside of the electronics andphotonic components for cooling. In addition, arrays of streamlinedparallel planar fluid flow structures 40 are provided within each recessto direct fluid flow close to base 22, and to increase fluid flow rateby means of the Bernoulli Effect.

Referring now to the second embodiment shown in FIGS. 6 to 11, similarparts to those of FIGS. 3 to 5 are denoted by the same referencenumerals. Referring firstly to FIG. 8, the outer container 20 has a topcover 27 of insulating material, with side flanges 29 which overlap base20, to completely envelope sealed container 24. In this case top 26 ofcontainer 24 may be thermally conductive. For clarity, top 27 is notshown in FIGS. 6 and 7.

In FIG. 6, recess 36 of FIG. 3 has been modified as a first recess 42beneath waveguides 12 separated by an intervening wall from a secondrecess 44 disposed beneath laser bar 14. Although the base 22 retainsthe hermetic seal of the first embodiment, the upper floor of eachrecess is formed with internal inlet and outlet fluid flow paths,directed to the electronics and photonics components. Thus in recess 38,see FIGS. 6 and 9, fluid flow paths 48 are provided for conducting fluidinto contact with electronics components. In recess 44, fluid paths 50are provided which are disposed adjacent laser bar 14.

In recess 42, fluid paths 52 are provided, disposed directly adjacentwaveguide structure 12. In recess 34, fluid paths 56 are provided whichare disposed directly adjacent optics 8. These fluid flow paths 48-56make sealed connections with fluid flow channels which are formed withinthe silicon of composite thermal substrate assembly 19 (by knownprocessing techniques), as shown in FIGS. 10 and 11 in particular. Thesefluid flow channels comprise a fluid flow network 62 coupled to paths 48for contacting the electronic components with cooling fluid. For laserbar 14, there are fluid input channels 64 in substrate 19 to conductfluid to the laser bar. Fluid outlet channels 66 permit fluid to flowaway from laser bar 14 back into recess 42 via paths 50.

Fluid flow paths 52 in recess 42 communicate with a reservoir 54disposed beneath AWG structure 10. Fluid flow paths 56 in recess 34communicate with a reservoir 58 disposed beneath free standing optics 8.

FIG. 12 shows the complete device 2 of the first or second embodiments,in position mounted on a circuit pack CP.

Advantages of embodiments are reduced power consumption, localizedthermal control for local component set points, internal componentthermal isolation from the circuit pack ambient, removal of thethermoelectric module, all while maintaining an hermetic seal and thenumerous input and output connections. Further advantages are: improvedthermal control—maintaining the correct temperature to ensure thecorrect optical wavelength; improved heat dissipation—extracting largerheat loads from smaller areas; improved thermal isolation—reducedthermal crosstalk across optical signals due to thermal differences,this aspect will improve transition times of operation and allow forgreater functionality of the optical components; removal of thethermoelectric module—significantly reducing the overall powerconsumption of the module; modular assembly—improving the isolation ofthe interior components from external influences which maintaining aclear heat dissipation path for efficient thermal operation.

The description and drawings merely illustrate the principles of theinvention. It will thus be appreciated that those skilled in the artwill be able to devise various arrangements that, although notexplicitly described or shown herein, embody the principles of theinvention and are included within its spirit and scope. Furthermore, allexamples recited herein are principally intended expressly to be onlyfor pedagogical purposes to aid the reader in understanding theprinciples of the invention and the concepts contributed by theinventor(s) to furthering the art, and are to be construed as beingwithout limitation to such specifically recited examples and conditions.Moreover, all statements herein reciting principles, aspects, andembodiments of the invention, as well as specific examples thereof, areintended to encompass equivalents thereof.

The invention claimed is:
 1. A thermal management apparatus for anelectrical and/or optical component assembly, comprising: anhermetically sealed container mounted within an outer container, theouter container having a liquid inlet and a liquid outlet, said outercontainer and said hermetically sealed container configured to define aliquid path operable to direct a liquid within the outer container andaround the hermetically sealed container to remove heat therefrom, andthe hermetically sealed container containing said component assembly,which assembly includes one or more devices, which devices are inthermal communication with a thermally conductive wall portion of saidsealed container, whereby said liquid cools said devices, wherein atleast two of the devices that are contained within the outer containerare also contained within the hermetically sealed container.
 2. Thethermal management apparatus according to claim 1 wherein the sealedcontainer has a base portion and side walls of a thermally conductivematerial.
 3. The thermal management apparatus according to claim 2,wherein a base and side walls of the outer container are formed of athermally insulative material.
 4. The thermal management apparatusaccording to claim 3, wherein the outer container is formed as a closefit to the sides of the sealed container, or is formed as plasticsextrusion moulded to the sealed container.
 5. The thermal managementapparatus according to claim 2, wherein the sealed container and theouter container have a common top portion formed of thermally insulativematerial, or have separate top portions of conductive and insulativematerial respectively.
 6. The thermal management apparatus according toclaim 1, wherein said outer container has liquid flow structures formedin said liquid flow path to direct liquid flow.
 7. The thermalmanagement apparatus according to claim 1, wherein the outer containerincludes a liquid inlet, which is coupled to a source of cooling liquid,and a liquid outlet.
 8. The thermal management apparatus according toclaim 1, wherein said one of more devices include a photonics device,preferably a laser bar comprising a plurality of lasers.
 9. The thermalmanagement apparatus according to claim 8, wherein said liquidpassageways include at least one liquid reservoir located beneath atleast one photonics device.
 10. The thermal management apparatus asrecited in claim 1, wherein a majority of the devices that are containedwithin the outer container are also contained within the hermeticallysealed container.
 11. A thermal management apparatus for an electricaland/or optical component assembly, comprising: an hermetically sealedcontainer mounted within an outer container, the outer container havinga liquid inlet and a liquid outlet, said outer container and saidhermetically sealed container configured to define a liquid pathoperable to direct a liquid within the outer container and around thehermetically sealed container to remove heat therefrom, and thehermetically sealed container containing said component assembly, whichassembly includes one or more devices, which devices are in thermalcommunication with a thermally conductive wall portion of said sealedcontainer, whereby said liquid cools said devices, wherein said one ormore devices are mounted on a thermally conductive substrate assemblywhich is sealed to and in good thermal contact with a base region of thesealed container.
 12. The thermal management apparatus according toclaim 11, wherein said substrate assembly is formed with liquid flowchannels which communicate with liquid inlets and outlets in the baseregion of the sealed container, in order to permit liquid flow from saidliquid flow path close to specific photonics devices.
 13. The thermalmanagement apparatus according to claim 11, wherein said substrateassembly is formed with liquid reservoirs which communicate with liquidinlets and outlets in the base region of the sealed container.
 14. Athermal management apparatus for an electrical and optical componentassembly, comprising: an hermetically sealed container mounted within anouter container, the outer container having a liquid inlet and a liquidoutlet, said outer container and said hermetically sealed containerconfigured to define a liquid path operable to direct a liquid withinthe outer container and around the hermetically sealed container toremove heat therefrom, and the hermetically sealed container containingsaid component assembly, which assembly includes one or more devices,which devices are in thermal communication with a thermally conductivewall portion of said sealed container, whereby said liquid cools saiddevices, wherein said outer container has liquid flow structures formedin said liquid flow path to direct liquid flow further comprising raisedliquid flow structures to encourage liquid flow at an increased rateclose to specific areas of the base of the sealed container.
 15. Thethermal management apparatus according to claim 14, wherein said raisedliquid flow structures are located in one or more recesses of the baseof the sealed container.
 16. Thermal management apparatus for aphotonics assembly, comprising an hermetically sealed container mountedwithin an outer container, and the hermetically sealed containercontaining a photonics assembly comprising one or more photonicsdevices, which are mounted on a thermally conductive substrate assembly,which is sealed to and in thermal contact with a wall portion of saidsealed container, the thermally conductive substrate assembly havingliquid passageways formed therein to permit liquid to flow close to saidphotonics devices, said wall portion having inlet and outlet liquidpaths communicating with said liquid passageways, and the outercontainer having a liquid inlet and a liquid outlet, so as to provide acooling liquid flow path between said liquid inlet and outlet via saidphotonics devices to remove heat therefrom.